This invention relates to power limiters and, more particularly, to power limiters for broad band infrared detector circuits.
Infrared detector circuits are often used in situations in which an overload may occur causing the detector to be damaged and become unusable. For example, infrared detectors are often able to operate in ranges in which the amount of power to be detected is unknown. They are quite able to cope with power up to approximately 10.sup.7 watts/cm.sup.2. However, if they encounter infrared signals over a threshold of approximately 10.sup.8 watts/cm.sup.2, the detector unit may well be damaged or destroyed.
One of the difficulties encountered in solving the overload problem is that the time necessary for reaction must be very short in order to protect the detector circuitry; consequently, normal feedback circuitry operating off the response of the detector circuitry does not operate rapidly enough in some situations to accomplish a power reduction before damage has been done to the detector. Thus, the limiter, in order to react most rapidly, should assess the power of the infrared beam directly. This form of sensitivity is unusual in power limiters.
Another difficulty in providing a power limiter for infrared detectors is that such detectors often must operate over quite broad bandwidths. Consequently, the power limiting arrangements must also operate over similar broad bandwidths.
In a U.S. patent application Ser. No. 820,848, entitled Liquid-Crystal-Based Optical Power Limiter, Shin-Tson Wu and David M. Pepper, filed coincident herewith by the same assignee, a solution is provided in which a family of power limiters are described which utilize the properties of liquid crystals to provide rapid protection over a broad bandwidth. In each, a liquid crystal is placed in the optical path through which the signals to be detected are to proceed. The liquid crystal is arranged so that in its normal condition the infrared signals are passed to the detector cell without any significant distortion or reduction in power. However, when an overload signal is detected the liquid crystal cell is distorted so that the infrared beam is defocused or directed other than along the path to the infrared detector cell. Since it is well known that liquid crystal cells may be made responsive over a very broad bandwidth, the invention provides for rapid protection of power overload for the infrared detector cell over a very broad bandwidth.
However, there are situations in which it is necessary to have the detector cell begin operating again as quickly as possible after an overload condition stops. Although the initial reaction time of the power limiters disclosed in the application mentioned above is sufficient to protect the cell, they often have too long a recovery time to be used in such situations.
It is, therefore, an object of this invention to provide a power limiter for protecting infrared detectors which has an extremely rapid recovery time.
It is another object of this invention to increase the overall speed of operation of power limiters.
An additional object of this invention is to provide a broad-band power limiter for light detectors with a rapid recovery time.